The lac Repressor mouse is the key and final component of a system for regulating gene expression in the mouse that is based on the lac operon of E. coli. The long-term goal of this project is to make available to the biomedical community at large a resource that will make it possible to introduce conditional lethal mutations into the mammalian genome and to analyze their phenotypic consequences within the context of an organism closely related to the human. Like analogous systems that use temperature sensitive mutations to study lethal mutations in bacteria and lower eukaryotes, this system would allow th introduction and analysis of embryonic lethal genes at the organismal level without compromising the viability of their host. The rudiments of the lac system ina the mouse are relatively simple; the creation of a line of Repressor mouse that expresses functional lac Repressor; and, modifications of the promoter region of a prospective transgene or endogenous locus so that it contains the DNA binding site for the Repressor (the lac operator). Transferred into a lac Repressor mouse background, such promoters become sensitive to regulation by Repressor in those tissues in which it is expressed. The Specific Aims of this project focus on acquiring information about when and in which tissues Repressor protein can be detected, and, more importantly, how well it functions to regulate gene expression at both the tissue and organismal levels. Such experiments will be carried out on two individual lines of homozygous lac Repressor mice in which Repressor transgenes have already been shown to be transcriptionally active. As described in the enclosed application, the technology required to regulate gene expression in the whole animal using regulatory elements from the lac operon of E. coli is on the verge of success. Theoretically, this technology would extend the possibilities of phenocopying human disease int eh mouse to include deleterious, loss-of-function mutations in essential genes that up to now have been inaccessible to experimental manipulation.